46.4 Gbps visible light communication system utilizing a compact tricolor laser transmitter.

Visible light communication (VLC), combining wireless communication with white lighting, has many advantages. It is free of electromagnetic interference, is rich in spectrum resources, and has a gigabit-per-second (Gbps) data rate. Laser diodes (LDs) are emerging as promising light sources for high-speed VLC communication due to their high modulation bandwidth. In this paper, we demonstrate a red/green/blue (R/G/B) LDs based VLC system with a recorded data rate of 46.41 Gbps, employing discrete multitone (DMT) and adaptive bit-loading technology to achieve high spectral efficiency (SE). The emission characteristics and transmission performance of R/G/B-LDs are discussed. The optimal data rates of R/G/B-LDs channels are 17.168/14.652/14.590 Gbps, respectively. The bit-error-ratio (BER) of each channel satisfies the 7% forward-error-correction (FEC) threshold (3.8×10-3) and greatly approaches the channel Shannon limit.

760 Mbps ultraviolet wireless communication based on a transmitter with AlGaN/InGaN MQWs.

III-nitride LEDs offer a solution for ultraviolet (UV) high-speed communication as a transmitter with high performance. This paper focuses on a transmitter with AlGaN/InGaN multi-quantum wells (MQWs) for UV communication. The transmitter is realized on a GaN-on-silicon platform by a double etching process. The emission region of the transmitter with a small area is beneficial for improving the data rate of UV communication. The emission peak keeps stable at 376.48 nm in the UVA band. The transmission with 300 Mbps is obtained in a UV communication system setup with on-off keying (OOK) modulation. We realize a digital signal transmission up to 760 Mbps by bit-loading discrete multi-audio (DMT) modulation.

Demonstration of flexible access in a rate-adaptive visible light communication system with constellation probabilistic shaping.

In this paper, we propose and experimentally demonstrate a distance-based rate-adaptive visible light communication (VLC) system based on constellation probabilistic shaping (PS) for a multiple-user access network. For users with different access distance, we optimize the transmission data rate close to the channel capacity by applying PS combined with code-rate adaptive FEC at the transmitter side according to the per-user signal-to-noise ratio (SNR) budget. This is also proved to be a convenient way to ensure fine granularity of information rate per user with wider flexibility compared with non-PS modulation formats. We also investigate the performances of different PS-QAM modulation formats under different SNR level when considering peak-to-average power ratio (PAPR) in the VLC system. Optimal PS-QAM and FEC code-rate are also studied in the flexible VLC access system. In addition, in order to overcome the nonlinear distortion in the system, a neural network (NN) is used as the post-equalization. Finally, we demonstrate the flexible access with the net data-rate from 1.84 to 3.37 Gbps for 20 and 1-meter distance, with a maximum 28% overall capacity improvement compared with regular non-PS modulations.

Improved electro-optical and photoelectric performance of GaN-based micro-LEDs with an atomic layer deposited AlN passivation layer.

The quantum efficiency of GaN-based micro-light-emitting diodes (micro-LEDs) is of great significance for their luminescence and detection applications. Optimized passivation process can alleviate the trapping of carriers by sidewall defects, such as dangling bonds, and is regarded as an effective way to improve the quantum efficiency of micro-LEDs. In this work, an AlN passivation layer was prepared by atomic layer deposition to improve the electro-optical and photoelectric conversion efficiency in GaN-based micro-LEDs. Compared to conventional Al2O3 passivation, the AlN passivation process has a stronger ability to eliminate the sidewall defects of micro-LEDs due to the homogeneous passivation interface. Our experiments show that the AlN-passivated device exhibits two orders of magnitude lower forward leakage and a smaller ideality factor, which leads to significantly enhanced external quantum efficiency (EQE). For 25*25 µm2 micro-LEDs, the EQE of the AlN-passivated device was 18.3% and 57.7% higher than that of the Al2O3-passivated device in luminescence application and detection application, respectively.

Toeplitz concatenated matrix aided ICA algorithm for super-Nyquist multiband CAP VLC systems.

In this paper, we demonstrate a Toeplitz concatenated matrix aided independent component analysis (TCM-ICA) equalizer that can skillfully compensate for the inter-channel interference (ICI) of super-Nyquist multiband carrierless amplitude and phase modulation (m-CAP) system. In the point-to-point super-Nyquist m-CAP visible light communication (VLC) scenario, we experimentally demonstrated a system-level average spectral efficiency (SE) enhancement of 0.50 b/s/Hz in comparison to the conventional scheme. In the multiple-input single-output (MISO) super-Nyquist 5-CAP scenario, the subcarrier-level Q factor enhancements of 4.4 dB, 5.2 dB, and 6.5 dB are achieved in comparison with the least-mean-square (LMS) post-equalizer for the 2nd, 3rd and 4th subcarrier, respectively. As far as we know, the TCM-ICA equalizer is the most effective ICI compensation scheme to enhance SE in the super-Nyquist m-CAP systems.

Enhanced performance of MIMO multi-branch hybrid neural network in single receiver MIMO visible light communication system.

For the single receiver multiple-input-multiple-output (SR-MIMO) visible light communication (VLC) system, the superposing of two transmitters will introduce severe distortion in the time-domain and frequency-domain. In this paper, we first proposed a MIMO multi-branch hybrid neural network (MIMO-MBNN) as the post-equalizer in the SR-MIMO pulse amplitude magnitude eight levels (PAM8) VLC system. Compared with the traditional single-input-single-output least mean square equalizer with Volterra series (SISO-LMS) and SISO deep neural network (SISO-DNN), MIMO-MBNN can achieve at most 3.35 dB Q factor improvement. Furthermore, the operation range of MIMO-MBNN is at least 2.33 times of SISO-DNN and SISO-LMS among the measured signal peak to peak voltage. At last, 2.1 Gbps data rate is achieved by MIMO-MBNN below the 7% hard-decision forward error correction (HD-FEC) threshold. As far as we know, this is the highest data rate in the SR-MIMO VLC system.

Underwater visible light communication at 3.24 Gb/s using novel two-dimensional bit allocation.

Underwater visible light communication (UVLC) systems suffer from a strong nonlinear effect and high inter-symbol interference (ISI). In this study, to improve the performance of a UVLC system under such conditions, we propose a novel nonlinear hybrid modulation scheme named two-dimensional bit allocation (2DBA). By comparing the performance of 2DBA with the famous Levin-Campello (LC) algorithm and the quadrature amplitude modulation (QAM)-based time-domain hybrid modulation (TDHQ) algorithm, we have proved by analysis and experiment that 2DBA can outperform the power allocation-based LC algorithm and the TDHQ algorithm below the 3.8×10-3 hard decision forward error correction threshold (HD-FEC) when the system has a severe nonlinear effect and ISI. The data rate 3.24 Gb/s of 2DBA is measured after 1.2 m underwater transmission; as far as we know, this is the highest data rate reported in a blue LED chip based UVLC system.

Comparison of nonlinear equalizers for high-speed visible light communication utilizing silicon substrate phosphorescent white LED.

Compared with multicolor-chip integrated white LEDs, phosphor-based white LEDs are more attractive for daily illumination due to lower cost and complexity, and thus they are preferable for future commercial use of visible light communication (VLC) systems. However, the application of phosphorescent white LEDs has a lower data rate than multicolor-chip integrated LEDs because of severe nonlinear impairments and limited bandwidth caused by the slow-responding phosphor. In this paper, for the first time we propose to employ phosphorescent white LEDs based on silicon substrate with adaptive bit-loading discrete multitone (DMT) modulation and a memoryless polynomial based nonlinear equalizer to achieve a high-speed VLC system. We also present a comprehensive comparison among nonlinear equalizers based on the Volterra series model, memory polynomial model, memoryless polynomial model and deep neural network (DNN) with experimental results utilizing a silicon substrate phosphorescent white LED, and provide detailed suggestions on how to choose the most suitable nonlinear mitigation scheme considering different practical conditions and the tradeoff between complexity and performance. Beyond 3.00 Gb/s DMT VLC transmission over 1-m indoor free space is successfully demonstrated with bit error rate (BER) under the 7% forward error correction (FEC) limit of 3.8×10-3. As far as we know, this is the highest data rate ever reported for VLC systems based on a single high-power phosphorescent white LED.

Gbit/s ultraviolet-C diffuse-line-of-sight communication based on probabilistically shaped DMT and diversity reception.

We demonstrated a high-speed 1×2 single-input and multiple-output (SIMO) diffuse-line-of-sight (diffuse-LOS) ultraviolet-C (UVC) solar-blind communication link over a distance of 5 meters. To approach the Shannon limit and improve the spectral efficiency, we implemented probabilistically shaped discrete multitone modulation. As compared to a single-input and single-output (SISO) counterpart, we observed significant improvement in the SIMO link in terms of the angle of view of the receiver and the immunity to emulated weather condition. A wide angle of view of ± 9° is achieved in the SIMO system, with up to a 1.09-Gbit/s achievable information rate (AIR) and a minimum value of 0.24 Gbit/s. Moreover, the bit error rate of the SIMO link in emulated foggy conditions is lowered significantly when compared to that of the SISO link. This work highlights the practicality of UVC communication over realistic distances and in turbulent environments to fill the research gap in high-speed, solar-blind communication.

Advanced Modulation Format of Probabilistic Shaping Bit Loading for 450-nm GaN Laser Diode based Visible Light Communication.

Visible light communication is an emerging high-speed optical wireless communication technology that can be a candidate to alleviate pressure on conventional radio frequency-based technology. In this paper, for the first time, the advanced modulation format of probabilistic shaping (PS) bit loading is investigated in a high data rate visible light communication system based on a 450-nm Gallium Nitride laser diode. The characteristic of the system is discussed and PS bit loading discrete multi-tone modulation helps to raise the spectral efficiency and improve the system performance. Higher entropy can be achieved in the same signal-to-noise ratio (SNR) and modulation bandwidth limitation, comparing to bit and power loading. With PS bit loading, an available information rate (AIR) of 10.23 Gbps is successfully achieved at the signal bandwidth of 1.5 GHz in a 1.2 m free space transmission with normalized generalized mutual information above 0.92. And higher AIR can be anticipated with an entropy-loading strategy that fixes the channel characteristic. Experimental results validate that a PS bit loading scheme has the potential to increase the system capacity.

Unveiling the State Transition Mechanisms of Ras Proteins through Enhanced Sampling and QM/MM Simulations.

In cells, wild-type RasGTP complexes exist in two distinct states: active State 2 and inactive State 1. These complexes regulate their functions by transitioning between the two states. However, the mechanisms underlying this state transition have not been clearly elucidated. To address this, we conducted a detailed simulation study to characterize the energetics of the stable states involved in the state transitions of the HRasGTP complex, specifically from State 2 to State 1. This was achieved by employing multiscale quantum mechanics/molecular mechanics and enhanced sampling molecular dynamics methods. Based on the simulation results, we constructed the two-dimensional free energy landscapes that provide crucial information about the conformational changes of the HRasGTP complex from State 2 to State 1. Furthermore, we also explored the conformational changes from the intermediate state to the product state during guanosine triphosphate hydrolysis. This study on the conformational changes involved in the HRas state transitions serves as a valuable reference for understanding the corresponding events of both KRas and NRas as well.

Mid-Infrared Bipolar and Unipolar Linear Polarization Detections in Nb2 GeTe4 /MoS2 Heterostructures.

Detecting and distinguishing light polarization states, one of the most basic elements of optical fields, have significant importance in both scientific studies and industry applications. Artificially fabricated structures, e.g., metasurfaces with anisotropic absorptions, have shown the capabilities of detecting polarization light and controlling. However, their operations mainly rely on resonant absorptions based on structural designs that are usually narrow bands. Here, a mid-infrared (MIR) broadband polarization photodetector with high PRs and wavelength-dependent polarities using a 2D anisotropic/isotropic Nb2 GeTe4 /MoS2 van der Waals (vdWs) heterostructure is demonstrated. It is shown that the photodetector exhibits high PRs of 48 and 34 at 4.6  and 11.0 µm wavelengths, respectively, and even a negative PR of -3.38 for 3.7 µm under the zero bias condition at room temperature. Such interesting results can be attributed to the superimposed effects of a photovoltaic (PV) mechanism in the Nb2 GeTe4 /MoS2 hetero-junction region and a bolometric mechanism in the MoS2 layer. Furthermore, the photodetector demonstrates its effectiveness in bipolar and unipolar polarization encoding communications and polarization imaging enabled by its unique and high PRs.

A two-dimensional mid-infrared optoelectronic retina enabling simultaneous perception and encoding.

Infrared machine vision system for object perception and recognition is becoming increasingly important in the Internet of Things era. However, the current system suffers from bulkiness and inefficiency as compared to the human retina with the intelligent and compact neural architecture. Here, we present a retina-inspired mid-infrared (MIR) optoelectronic device based on a two-dimensional (2D) heterostructure for simultaneous data perception and encoding. A single device can perceive the illumination intensity of a MIR stimulus signal, while encoding the intensity into a spike train based on a rate encoding algorithm for subsequent neuromorphic computing with the assistance of an all-optical excitation mechanism, a stochastic near-infrared (NIR) sampling terminal. The device features wide dynamic working range, high encoding precision, and flexible adaption ability to the MIR intensity. Moreover, an inference accuracy more than 96% to MIR MNIST data set encoded by the device is achieved using a trained spiking neural network (SNN).